<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>外部css</title>
        <link type="text/css" rel="stylesheet" href="./extend.css"/>
    </head>
    <body>
        <div>
            标签选择器
        </div>
        <div id="first-div">
            id选择器
        </div>
        <div class="bg-div">
            class选择器
        </div>

    </body>
</html>